1. Field of the Invention
The present invention relates to a superconductor filter, particularly, to an improvement in the coupling of a resonance element included in a superconductor high frequency filter.
2. Description of the Related Art
A high frequency filter is incorporated as a main part in communication equipment for wireless communication or communication through wire of information. The high frequency filter, which performs the function of filtering a desired frequency band alone, is a functionally important constituent of the communication equipment. In order to operate the communication equipment in a more energy-efficient fashion by effectively utilizing the frequency, the high frequency filter is required to be good in the attenuation characteristics and to be small in the insertion loss. In order to prepare a filter meeting these requirements, it is necessary to obtain a resonance element having a high Q value. As a method for realizing a resonance element having a high Q value, it is proposed in recent years to use a high temperature superconductor material, which is a material having a very small surface resistance, as a conductor constituting the resonance element.
In the structure of a high frequency filter formed of a superconductor thin film, a half-wave resonance element or the like is formed by a distributed constant circuit such as a micro strip line on a substrate. In general, these resonance elements are arranged to form a multi-stage structure and are spatially connected to each other.
In a high frequency filter, the resonance elements are spatially coupled to each other by electromagnetism so as to determine the filter characteristics. Therefore, generally, varying the relative positions, at which the resonance elements are arranged, is used as the standard method of design. In other words, the filter is designed such that the adjacent resonance elements are arranged closely in the case where a strong coupling is required or further apart in the case where a weak coupling is required.
The Chebyshev function type filter, which is known as a typical filter structure, is constructed by utilizing the electromagnetic coupling alone between the adjacent resonance elements. In the Chebyshev function type filter, the resonance elements are linearly arranged such that a relatively large distance is provided between a certain resonance element and another resonance element other than the resonance element positioned adjacent to said certain resonance element, so as to make it relatively difficult for an undesired coupling to take place.
On the other hand, a pseudo elliptical function type filter is disclosed on page 1656 of “IEEE Transactions on Microwave Theory and Techniques, Vol. 47 (1999)”. The pseudo elliptical function type filter is constructed such that a certain resonance element, i.e., a first resonance element, is intentionally coupled with a resonance element other than the resonance element positioned adjacent to the first resonance element, which is called a jumping coupling, for planarizing the group delay characteristics. Also disclosed on page 1656 of “IEEE Transactions on Microwave Theory and Techniques, Vol. 47 (1999)” is a method for achieving the adjacent coupling and the jumping coupling by utilizing the spatial coupling.
On the other hand, disclosed on page 661 of “IEEE Microwave Theory and Techniques Symposium Digest (2000)” is a method in which the spatial coupling is employed for the coupling of the adjacent resonance elements and a coupling transmission line, i.e., a transmission line for the coupling, is employed for the jumping coupling. In the prior art disclosed in this literature, the resonance elements are linearly arranged, and the resonance elements are arranged relatively far away from each other, except for the adjacent resonance elements.
As described above, the Chebyshev function type filter is constructed such that a relatively large distance is provided between the resonance elements other than the adjacent resonance elements so as to make it relatively difficult for an undesired coupling to take place. However, there is a lower limit in the distance between the resonance elements. It is impossible for the distance between the resonance elements to be zero, except for the distance between the adjacent resonance elements. It should be noted that the coupling between the resonance elements other than the coupling between the adjacent resonance elements gives rise to the problem that the actual filter characteristics deviate from the desired filter characteristics. To be more specific, it is necessary to redesign or adjust the arrangement of the resonance elements in an attempt to obtain the desired characteristics.
In the method of forming the adjacent coupling and the jumping coupling by using the spatial coupling, which is disclosed on page 1656 of “IEEE Transactions on Microwave Theory and Techniques, Vol. 47 (1999)” referred to above, the resonance elements that are originally irrelevant to each other in respect of the coupling are positioned close to each other in the process of forming a jumping coupling. As a result, a serious problem is generated that an undesired coupling is generated between the resonance elements positioned close to each other.
In the method disclosed on page 661 of “IEEE Microwave Theory and Techniques Symposium Digest (2000)” referred to above, a spatial coupling is employed for the coupling of the adjacent resonance elements, and the coupling transmission line is employed for the jumping coupling. In this method, a relatively large distance is provided between a certain resonance element and another resonance element other than the resonance element positioned adjacent to the said certain resonance element so as to make it relatively difficult for an undesired coupling to take place. However, there is a lower limit in the distance between the resonance elements. It is impossible for the distance between the resonance elements to be zero except the distance between the adjacent resonance elements. It should be noted in this connection that the filter of this construction gives rise to the problem that the actual filter characteristics deviate from the desired filter characteristics. To be more specific, it is necessary to redesign or adjust the arrangement of the resonance elements in an attempt to obtain the desired proper characteristics.